The present invention relates to a loop-structured communication system. More prticularly, to a loop-structured communication system that is capable of disconnecting a selected loop section and rearranging the loop while maintaining synchronism of data stream on the loop.
Communication systems in the form of a closed loop in which numerous remote terminals are interconnected have the advantage of a saving in transmission lines and a favorable organization of the data traffic with the aid of time frames circulating on the loop. One problem present in such systems is the fact that when an interruption occurs in the communication loop, the total system is placed out of operation. Such interruptions may be caused by mechanical or electrical failures or by a disconnection of a loop section for maintenance or repair.
Systems are presently known in the art, which address the above problem by loop reconfiguration, thereby maintaining operability of the system with the exception of that part of the loop in which the interruption has occurred. In one of these prior art systems the communication loop consists of parallel lines on which data signals can circulate in opposite directions. The connection units at each terminal in this system provide the additional function of a switchover unit in which data signals can either be forwarded on the same line in the same direction or in which they can be retransferred from one line to the other line. Normally, data signals are transmitted over only one of these lines. If an interruption occurs the data stream is reversed in the connection units before and after the location of the interruption, so that with the aid of the second parallel line two folded loops are set up which together again form a closed loop.
In several other prior art systems, isolation of part of a communication loop with reconfiguration of the remaining closed loop is achieved with the aid of switchover units used in a similar manner as the connection units.
In all these prior art systems the data stream is temporarily interrupted due to the disconnection of a loop section, and part of the attached functional units or terminals are caused to fall out of synchronism. After completion of the new configurated loop the units must again be resynchronized and the system must determine which data was lost.
While such a procedure may be tolerated in some applications it is not acceptable in others. For example, in many process control systems data must be accepted or furnished within fixed time intervals. Otherwise, serious damage could result in the controlled facility.